High speed and high capacity transmission technologies are becoming essential due to recent spread and growth of mobile phones and other wireless communications. In order to realize high speed and high capacity communications, it is necessary to ensure a broad frequency band. A frequency band used by wireless communications is shifting to high frequencies. In such a high frequency band, filters for wireless communications are required to have filter characteristics capable of selectively passing only desired frequency components and steeply cutting other frequency components. A wireless communication instrument using high frequency components is required to be made compact and light.
As a filter used in a high frequency band, a filter is known which includes a ring resonator constituted of a ring-shaped transmission line. A resonance frequency of a ring resonator is specified by a transmission line length of the resonator. A resonance wavelength or an integer multiple thereof is equal to the electric line length of the ring resonator. In order to increase a space efficiency of a ring resonator, a structure has been proposed by which one ring resonator is resonated in two resonance modes (dual mode) to obtain steeper filter characteristics.
An input line and an output line are coupled with a ring resonator at two points separated from each other by a transmission line length corresponding to a quarter wavelength. By disposing a stub between two coupling points, resonance in dual mode is generated. Resonance in dual mode is also generated by disposing a distributed coupled line outside a ring resonator along a partial circumference of the ring resonator. This distributed coupled line is disposed running in parallel to the ring resonator at generally a middle point of a longer one of two transmission lines, both ends of which correspond to the two coupling points being coupled with the input and output lines.
The following are examples of related art of the present invention: Japanese Patent Laid-Open No. 9-139612, Japanese Patent Laid-Open No. 2000-209002.